_keys is defined as a class attribute in FakeFlavorResource. So when
we call set_keys() to update it, it changes. And this change may bring
trouble to the other tests afterward.
So define and initialize it in __init__() as an object attribute.
Change-Id: Ib18c03877b67e1b7c2e107f598076b928a58e4fb
Closes-bug: #1548378